The present invention is in the area of hand-held striking tools, such as hammers, and pertains more specifically to the interface between head and handle of a striking tool.
Hand-held striking tools have been used for centuries by people in a variety of disciplines as leveraged devices to provide a striking force to accomplish a seemingly endless variety of tasks. For example, a claw hammer allows a user doing carpentry work to deliver sufficient striking force to drive a nail into wood. A claw hammer is also used for removing a nail or ripping apart lumber using its claw. A sledge hammer is another common hand-held striking tool used to deliver sufficient striking force for heavy work such as driving a stake, chisel, or driving a wedge into masonry, stone, wood, or other hard materials. Another common hand-held striking tool is a ball peen hammer used to deliver sufficient striking force for shaping and fitting metal, and for driving machine chisels, rivet sets, machine wedges, and other similar tools.
As previously described, hand-held striking tools are commonly used as third class levers to provide a striking force to accomplish tasks such as driving a nail into a piece of wood, bending or forming metal, breaking a rock, and other similar tasks. Third class levers are levers where a fulcrum, also referred to as a pivot point, is at one end of a bar or rod. A load to be overcome is an object creating resistance at the opposite end of a bar or rod. The effort, or force, to be applied to a third class lever is somewhere in between the fulcrum and load. In the case of a hand-held striking tool such as a claw hammer, the fulcrum is a wrist with the force being provided by the deceleration of the movement of a hammer handle (bar or rod) at the wrist. The load in this case is a resistance presented by a piece of wood into which the nail is being driven.
The head of the hand-held striking device is commonly a significant distance from the fulcrum and moves faster than the movement being applied at a user""s hand, which is near the fulcrum. The increased speed of the head multiplies the applied force with which a striking device head strikes a nail or digs into the dirt. The longer a claw hammer""s handle, for example, the faster the head and the greater the force that strikes a nail and overcomes the resistance of the wood. This principle applies to all other hand held striking devices, and is intensified in long-handled striking devices such as a pickaxe, or an axe.
Hand-held striking tools are also commonly used as first-class levers to provide a lifting or prying force to accomplish a variety of tasks. For example, some hand-held striking devices are used to pull nails out of a piece of wood, tear apart pieces of wood or other building material, pry loose a large rock, lift a log, and the like. First-class levers are levers wherein the load to be overcome is at or near one end of a rod or bar, the effort, or force is applied at or near the other end of the same rod or bar, and the fulcrum, or pivot, is somewhere along the rod or bar in between the applied force and load.
An example of a hand-held striking tool being used as a first-class lever is a claw hammer being used to pull out nails, wherein the load to be overcome is the wood causing friction against an embedded nail. Another example of a hand-held striking tool being used as a first-class lever is a pickaxe being used to pry out a rock or tree root embedded in dirt or rock, where the load to be overcome is the dirt or rock causing friction against an embedded rock or tree root. Whenever a hand-held striking tool is used as a first-class lever, the force is applied at one end of a long handle. The fulcrum is typically near the other end of the handle that holds the head.
The load for a hand-held striking tool being used as a first-class lever, such as in a claw hammer or a pickaxe, is typically very close to the fulcrum. Whereas the force for a hand-held striking tool being used as a third-class lever is typically relatively far away from the fulcrum. During prying or pulling tasks, the load applied is therefore moved less distance than the hand, which is at the opposite end of the lever, and applying the force. This multiplies the force in which the claw hammer head pulls against a nail, or a pickaxe pulls against a rock.
The weakest part of a hand-held striking device is the interface between the handle and the head. The conventional methods of interfacing a striking device head and handle, which are typically made of distinct materials, such as metal and wood, allows striking and pulling stresses to promote head-to-handle loosening, damage, and separation. For example, the impact force at the head of a claw hammer, being used as a third-class lever against a nail, is often as high as 300 pounds. Because of the greater length of its handle and greater weight of its head, the striking force of the head of a pickaxes against the earth is many times greater.
The bending moment applied at the head-to-handle interface of a claw hammer been used as a first-class lever to pull a nail is often as high as 1000 foot-pounds. The bending moment levied against the head-to-handle interface of a pickaxe pulling heavy rocks away from the earth is typically several times more.
The effect of these forces is exacerbated when a user occasionally misses the target for which the strike is intended and strikes a hard object, such as the edge of a piece of wood or a rock, at the head-to-handle interface just below the head, causing further damage and weakening a head-to-handle interface.
Because of the inherent weakness in conventional head-to-handle interfaces, it is this point that most failures in hand-held striking devices occur. Methods have been devised to make head-to-handle interface configurations capable of withstanding impacts and pulling stresses described above without damage. These methods include using a handle made with a material, such as high-impact plastic or heavy-gauge rolled steel that has particularly high strength and resiliency to withstand extremely high impacts and pulling stress. These types of handles are typically encapsulated in a resilient material, such as natural or synthetic rubber, leather, or plastic, to provide some protection from the shock from impact and to give a user a good grip on the handle. Many users of hand-held striking devices, however, still prefer the look and feel of wooden handles.
As stated above, a problem with many conventional methods for increasing handle strength on hand-held striking devices is the inherent weakness in the design of interfaces. Current interfaces for hand-held striking tools typically comprise a handle whose end is shaped to make a tight fit through a shaped opening, or eye, in the head. Such a shaped opening is often tapered to be larger at the end of the opening opposite the side where the handle enters the head. Typically metal or wooden wedges are driven into the end of the handle to expand the handle into the eye of the head to attain a tight fit. A tight fit, however, does little to increase the strength of the conventional head-handle interface.
Another common method for securing conventional head-to-handle interfaces is by placing a bonding material, such as an epoxy adhesive, between the inner surface of the opening in the head and outer surface of the interface and of the handle.
The types of head-to-handle interfaces and methods of securing described above are commonly used on all types of hand-held striking tools, such as axes, sledge hammers, pickaxes, and the like. A problem with these conventional solutions is that the striking and pulling forces are concentrated over a short distance at the interface. The intensified stress at this small area is the cause of most hand-held striking tool failures. Head-to-handle interfaces made according to conventional art, regardless of the material of the handle or method of securing it to the head opening, often fail because of this concentrated stress.
What is clearly needed is an improved method and apparatus for securing the head of a striking tool to its handle, providing both a durable, long-lived head-to-handle interface and an integrated system of overstrike protection to the handle. Such a method and apparatus, in a preferred embodiment, overcomes an inherent weakness in conventional head-to-handle interface methods while preventing damage to the handle caused when a user misses a target and inadvertently strikes the handle against a hard object, damage that causes, further weakness in conventional head-to-handle interface methods.
In a preferred embodiment of the present invention, for a striking tool having a plane of substantial symmetry and a head with an eye for engaging a portion of a handle, a method for securing the handle to the head is provided, comprising the steps of (a) slotting the portion of the handle that extends into the eye of the head in the plane of substantial symmetry, providing a slot with a first width; (b) engaging a metal strongback unit in the slot, the strongback unit having a web with a thickness substantially the width of the slot; (c) engaging the handle and strongback assembly into the eye, and (d) spreading the web of the strongback unit until metal of the strongback unit urges against an inside surface of the head in the eye.
In a preferred embodiment the web has at least one opening at an upper edge for a spreading wedge, and the web is spread in assembly by driving a wedge into the handle and the opening in the web. The strongback unit may also comprise side elements having a second width greater than the web width, such that the side elements lie along opposite sides of the handle with the strongback unit engaged in the handle slot. Spreading the web urges the side elements against the inside surface of the head in the eye. Also in a preferred embodiment the side elements are shaped to fit a curvature of the inside surface of the head in the eye. In some cases the handle is formed on opposite sides of the slotted portion to accommodate the side elements of the strongback unit.
In another preferred embodiment the slot extends further along a length of the handle than the portion of the handle that extends into the eye of the head, and the strongback unit fills the slot, such that the side elements, with the handle and strongback engaged, provide an overstrike surface along a portion of the handle length below the head. In this embodiment as well, the handle may be formed along opposite sides along the slot edges to accommodate the side elements.
In still another preferred embodiment the strongback unit has a first web in an upper portion having a length equal to the handle portion extending into the eye, and a second web in a lower portion, the second web formed at a right angle to the first web, and wherein the handle is slotted with first and second slots at right angles to accommodate both webs. In this embodiment the second web comprises threaded holes, and an overstrike plate is assembled to the handle below the head on the side of the handle toward a striking surface of the head, the overstrike plate secured to the handle by fasteners extending through holes in the plate and the handle, and engaging the threaded holes in the lower web.
In another aspect of the invention a striking tool having a plane of substantial symmetry is provided, comprising a head with an eye for engaging a portion of a handle; a handle with a slot of a first width in the plane of substantial symmetry along a length of the portion of the handle extending into the eye; and a metal strongback unit in the slot, the strongback unit having a web with a thickness substantially the width of the slot. The striking tool is characterized in that the strongback unit is spread in the eye, urging metal of the strongback unit against inside surface of the head in the eye, to secure the head on the handle. In preferred embodiments the web has at least one opening at an upper edge for a spreading wedge, and the web is spread in assembly by driving a wedge into the handle and the opening in the web.
In some embodiments the strongback unit comprises side elements having a second width greater than the web width, such that the side elements lie along opposite sides of the handle with the strongback unit engaged in the handle slot. Spreading the web urges the side elements against the,inside surface of the head in the eye. The side elements may be shaped to fit a curvature of the inside surface of the head in the eye. Also, the handle may be formed on opposite sides of the slotted portion to accommodate the side elements of the strongback unit.
In another embodiment the slot extends further along a length of the handle than the portion of the handle that extends into the eye of the head, and the strongback unit fills the slot, such that the side elements, with the handle and strongback engaged, provide an overstrike surface along a portion of the handle length below the head. The handle may be formed along opposite sides along the slot edges to accommodate the side elements.
In yet another preferred embodiment the strongback unit has a first web in an upper portion having a length equal to the handle portion extending into the eye, and a second web in a lower portion, the second web formed at a right angle to the first web, and wherein the handle is slotted with first and second slots at right angles to accommodate both webs. The second web may comprise threaded holes, and an overstrike plate may be assembled to the handle below the head on the side of the handle toward a striking surface of the head, the overstrike plate secured to the handle by fasteners extending through holes in the plate and the handle, and engaging the threaded holes in the lower web.
In various embodiments of the present invention described in enabling detail below, an improved apparatus and method is provided for securing handles to heads of striking tools.